1. Field of the Invention
The present invention relates generally to tunable lasers and other tunable optical signal sources and, more particularly, to a method and apparatus for automatic closed-loop gain adjustment in a closed-loop control system containing a temperature tuned, wavelength stabilized laser source or other tunable optical signal source.
2. Description of the Related Art
Optical fiber communications systems provide for low loss and very high information carrying capacity. In practice, the bandwidth of optical fiber may be utilized by transmitting many distinct channels simultaneously using different carrier wavelengths. The wavelengths of the various channels used in communication systems have been standardized by the International Telecommunications Union (ITU) grid. The ITU grid includes frequencies from approximately 191,900 GHz to 195,800 GHz with a separate channel occurring approximately every 100 GHz. The associated technology is called wavelength division multiplexing (WDM). Dense WDM, or DWDM, systems incorporate 25 or 50 GHz channel separation.
The wavelength bandwidth that any individual channel occupies depends on a number of factors, including the impressed information bandwidth, and margins to accommodate carrier frequency drift, carrier frequency uncertainty, and to reduce possible inter-channel cross-talk due to non-ideal filters.
To maximize the number of channels, lasers with stable and precise wavelength control are required to provide narrowly spaced, multiple wavelengths. Various approaches have been used to limit the oscillation of a laser to one of the competing longitudinal modes. One approach is a distributed feedback mechanism. Distributed-feedback (DFB) lasers are the most common type of communications laser. A grating integral to the laser structure limits the output to a single frequency. Another of the most common methods is temperature tuning, which requires the use of a frequency selective external cavity/etalon in combination with such a laser device to detect the output wavelength, based on the etalon""s response curve, at which the laser is operating. The laser frequency can be determined based on the normalized etalon output and adjusted accordingly by varying the temperature of the laser. Such a method allows for wavelength locking of a laser even in the event of changing ambient temperature conditions.
Etalon based wavelength stabilized laser sources are rapidly becoming preferred within the optics field. Systems incorporating an etalon based wavelength stabilized laser source typically use a feedback loop to vary the temperature of the laser (based on the normalized etalon output) and are sometimes referred to as closed-loop feedback control systems. These systems typically have a life of 25 years.
Typical closed-loop feedback systems feed back a control signal to adjust the temperature of the laser after variations in the wavelength of the laser optical output has been detected. As noted above, these systems use an etalon output and a reference output to determine the wavelength of the laser output. The etalon output is dependent upon the optical amplitude and wavelength of the laser output. On the other hand, the reference output is dependent solely on optical amplitude. Changes in the optical amplitude of the laser output entering the etalon filter creates changes in the periodic slope values of the etalon response curve. The changes in the periodic slope occur even though the etalon response curve will retain its general shape and periodicity as a function of wavelength. This is typically not a problem after the output has been normalized to a reference signal because under normal circumstances, after normalization, a change in optical power alone will not produce a change in slope for the same laser. However, if an external physical mechanism has altered the response characteristics of the etalon, a change in slope may occur, even after normalization. This change in slope may be a problem (described below).
FIG. 1 illustrates two exemplary etalon response curves 10, 14, each representing etalon outputs that have been converted into electrical signals from optical signals, and their respective reference outputs 12, 16. FIG. 1 illustrates two etalon curves 10, 14 that have been generated from two separate laser outputs into the same etalon filter, each laser output having a different optical amplitude. The two etalon curves 10, 14 are illustrated in the same graph for comparison purposes only. Although the curves 10, 14 have different optical amplitudes, they both retained their general shape and periodicity as a function of laser wavelength (and temperature as will be described below).
Each reference output 12, 16 has an associated reference amplitude level. In determining the wavelength of the laser output, the etalon curves 10, 14 are normalized with respect to their respective reference outputs 12, 16. FIG. 2 illustrates normalized etalon curves 20, 24 and a reference output 22. Again, FIG. 2 illustrates two normalized etalon response curves 20, 24 for comparison purposes only. As known in the art, a normalized etalon output, e.g., curve 20, is used to calculate the control signal used to control the temperature of the laser. Some gain value must be applied to properly adjust the feedback control signal relative to the amount of change that has occurred in the etalon output. The actual gain value used is system dependent and is usually determined and stored when the system is initially calibrated. After the system is calibrated, and the gain value is determined, the gain value is not adjusted.
It is known that the value of the gain used to generate the control signal is dependent solely upon the slope of the etalon response curve when the change in the etalon response has occurred. If the system is properly calibrated, the slopes of the system will be known, gain values can be set accordingly and used to generate proper feedback control signals. Unfortunately, these slopes may unexpectedly change after the system is calibrated. Possible causes for the changing slopes include, but are not limited to, aging or changes in the optical coatings on the etalon filter or a change in the alignment of device optics within the laser. These unexpected and unaccounted for increases or decreases in the etalon response curve slopes, without an adjustment of the closed-loop gain of the system, may result in strong oscillations of the control signal about the desired wavelength locking point. This can also cause the system to respond to changes in laser wavelength too slowly; thus, risking wavelength drift beyond the system specifications. This may even cause mode-hopping of the laser wavelength to an entirely different channel.
Current systems use power control circuitry to re-adjust the optical power of the laser to a known constant output value when there is a change in optical output power. Unfortunately, the power control/recovery loop may not respond within the time necessary to prevent the above-mentioned problems. This is due in part because the temperature control loop is constantly engaged and reacting at a very fast speed relative to the power control loop. There is another problem with these systems. That is, in some of the devices, such as the distributed Bragg Reflector (DBR) laser, the power control loop does not make use of the optical signal being output from the backface of the laser (often referred to as the xe2x80x9cbackface outputxe2x80x9d), which is the signal typically used to control wavelength locking. Instead, these systems will vary an amplifier or other device from the front face of the laser (i.e., the main laser output that does not pass through an etalon filter). As such, the etalon will not see the power recovery. Accordingly, there is a desire and need for automatic closed-loop gain adjustment in a closed-loop feedback control system containing a temperature tuned, wavelength stabilized laser source or other tunable optical signal source.
The present invention provides a method and apparatus for automatic closed-loop gain adjustment in a closed-loop feedback control system containing a temperature tuned, wavelength stabilized laser source or other tunable optical signal source.
The above and other features and advantages of the invention are achieved by providing a closed-loop feedback control system with a temperature tuned, wavelength stabilized laser module. The system uses a feedback control signal dependent upon a predetermined gain to control the temperature of the laser module. The laser module has an output connected to a controller via filtering and reference circuitry. The controller inputs etalon and reference signals from the filtering and reference circuitry to monitor the optical amplitude and wavelength of the laser module output, as well as the temperature of the laser module. When the controller detects a change in a slope of the etalon signal, the controller calculates a new numeric gain based on the changed slope. The new numeric gain and temperature of the laser module is used to generate a new control signal to maintain the output of the laser module at a desired wavelength. By monitoring the slope of the etalon signals, the system is capable of performing automatic closed-loop gain adjustment. As such, the system prevents strong oscillations of the control signal about a desired wavelength locking point, wavelength drift beyond system specifications and unwanted mode-hopping of the laser wavelength to undesired channels.